Cycles Netrender Returning Black
I don't know how to fix your problem with netrender, but a possible alternative method you could use is to tell each server to write to a shared directory, enable Placeholders and disable Overwrite.

With Overwrite disabled, each server will check to see if a file already exists where it would otherwise write the next frame. If it does, it checks the next frame, and the next, and the next, until it finds a frame where no file exists. Then it will render that frame and write it.

Enabling Placeholders makes the server write a "dummy" file when it starts rendering to "reserve" that filename for the render when is complete. This ensures that one server doesn't start rendering the same frame another server is already rendering but hasn't written yet.